The present invention relates to a slurry for CMP (Chemical Mechanical Polishing) and a CMP method which are effective in grinding a metal film such as an Al film, a W film or a Cu film.
In these years, in the field of fabrication of semiconductor devices, various microfabrication techniques have been researched and developed to promote microfabrication and higher integration concentration. Among them, CMP techniques are very important in forming Damascene wiring.
In conventional metal CMP techniques, a slurry containing alumina abrasives as a basic component is used. The reasons are that alumina itself has a high polishing performance and alumina abrasives have good dispersivity in a strong acid (pH: ˜2) such as nitric acid, which is widely used as an oxidizing agent.
The alumina-based slurry, however, has three problems.
First, in order to polish a metal film such as a Cu film or an Al film, the pH of the slurry needs to be set at an approximately neutral value. However, since alumina-based slurry has unstable dispersivity near the neutral value, it is difficult to obtain stable polishing characteristics.
Second, an adequate selectivity ratio of a metal film to an insulating film (i.e. metal film polishing rate/insulating film (SiO2) polishing rate) cannot be maintained. Where Damascene wiring is to be formed, it is necessary to maintain an adequate selectivity ratio of a metal film to an SiO2 film. In general, for this purpose, the alumina concentration is reduced to lower the SiO2 film polishing rate. However, the metal film polishing rate is also reduced at the same time. Consequently, it is difficult to obtain a high selectivity ratio, and thinning of the SiO2 film cannot be suppressed.
Third, dishing (formation of a dish-like concave in a metal film within a trench) will occur to a great degree. In particular, it is not possible to form Damascene wiring with a small over-polishing margin and a uniform depth over an entire surface of a wafer. To overcome this problem, a hard polishing pad with low elastic deformation is used. In the case of a soft material such as aluminum, however, the use of the hard polishing pad requires a trade-off between reduction of dishing and occurrence of flaws. Therefore, unless the slurry is devised, this problem is difficult to solve.
There is an attempt to carry out metal CMP using a slurry containing silica abrasives as a main component. In this case, however, an adequate polishing rate is not achieved, and morphology of a polished face is not good.
As has been described above, in the conventional metal CMP, the alumina-grain-based slurry has been used. Where this kind of slurry is used, it is difficult to obtain stable polishing characteristics near the neutral value of pH or to maintain an adequate selectivity ratio of a metal film to an insulating film. Moreover, dishing will occur to a great degree.